I need a spiral shaped like this. It's an approximation created using bezier curves, but I would guess there is a generic formula for shape like this.

Important properties:
- 3x symmetry axis
- I thing that the tangent of the curve at the point where it leaves the 60deg piece is the same as the tangent of the points where curve enters the same 60deg piece. I am not sure how to describe it more clearly.

How can I find it?